Spirits appear in previously cleared room like Fiends as well any room in the Spiritual Dungeon besides the final boss room. Spirit will run away from you in the opposite direction until they reach a wall and dissapear. Approximately 0.966667 spirits spawn per room.
They always drop a potion, in about 2/3 or 3/4 of the cases a health potion, otherwise a mana potion. (Tested for low level.)